Field of the Invention
The present invention concerns a method for determining deformation information in an examination object that executes a cyclical movement, and to a magnetic resonance (MR) system for implementing such a method.
Description of the Prior Art
When MR images of a moving organ are recorded, such as heart or the liver, the organ's own motion must be taken into account, and possibly the movement of the entire organ due to the movement of the surrounding area. One possibility for imaging moving objects is known as the single-shot technique, in which the raw data space of an associated MR image is entirely read out after radiating a single RF pulse sequence and in which the recording of the MR data is sufficiently fast to freeze the movement. In a further recording technique known as segmented recording, the data recording for an MR image is divided over several movement cycles and the MR data are recorded only in comparable movement phases. During heart imaging, the breathing and the heart movement must be taken into account, and the movement can be minimized by means of a breath-hold technique or can be frozen by a navigator gating. A further possibility for recording such data is known as the cine data recording for measuring the myocardial muscle movement, in which a number of MR images per heart cycle are recorded with as good a contrast as possible between the myocardial muscle and blood, so that a type of movie of the heart movement is produced.
One parameter of interest when recording moving examination objects is the determination of a deformation of the moving examination object. For instance, a deformation in the peripheral direction of the myocardium and an item of radial deformation information can be determined as deformation information of the myocardium. A conclusion as to the vitality of the myocardium in the corresponding areas can be made from the deformation information in individual areas of the myocardium. During the recording of the myocardium in the short axis section, it is nevertheless difficult to calculate a deformation of the myocardium in the peripheral direction for different segments of the myocardium, because sufficient markers are not present in the myocardium on the basis of which a deformation can be estimated in the peripheral direction. This applies as well to the longitudinal deformation in the longitudinal axis sections of the heart.
It is also known to subject the magnetization to a spatial magnetization pattern such as a grid pattern. With such methods, also known as tagging methods, the examination object appears with a strip or grid pattern in the MR image. This pattern can also be used to determine the deformation information, but a radial determination of the deformation information is difficult, because there are not sufficient marker points in the radial direction.
When determining the deformation information of the myocardium or any other moving examination object, the myocardium must be reliably identified in the MR images and separated from other tissue. The segmentation of the moving object such as the myocardium that is required in the MR images is particularly difficult in MR images in which a spatial magnetization pattern was applied with the tagging method.